PROJECT SUMMARY Significance: As many as one third of people living with HIV in sub-Saharan Africa experience virologic failure within two years of initiating antiretroviral therapy (ART). However, it is difficult to discern the etiology of treatment failure as adherence-driven versus resistance-driven in resource-limited settings (RLS). Genotypic resistance testing is not routinely available due to high costs and limited laboratory infrastructure in many settings. Adherence assessment is similarly challenging because self-reported measures have low validity, and currently available objective measures require costly equipment and/or specialized personnel. However, a novel immunoassay to measure tenofovir (TFV) concentrations in urine has been recently developed and is being translated into a point-of-care adherence assay for use in the context of pre-exposure prophylaxis. Innovation: Herein, we propose the first study to validate this recently developed urine TFV immunoassay in a population of people living with HIV on treatment. Furthermore, we will evaluate the immunoassay as a pathway to a low-cost, point-of-care method to distinguish adherence-driven from resistance-driven treatment failure. Investigators: Our interdisciplinary team, with expertise in HIV resistance and epidemiology (Principal Investigator Suzanne McCluskey, an early career investigator fitting the NIH's ?Next Generation Investigator? goals), leading clinical trials in sub-Saharan Africa (Mark Siedner), HIV clinical care (Mwebesa Bwana), pharmacologic adherence measures in HIV (Monica Gandhi), virology (Pravikrishnen Moodley), analytical chemistry (John Adamson) and biostatistics (Bethany Hedt-Gauthier), will conduct the aims of this grant. Approach: We will leverage the infrastructure of the REVAMP study, an NIH R01-funded clinical trial (R01 AI124718) in Uganda and South Africa. REVAMP enrolls individuals failing first-line ART in the two countries, most of whom are on TFV-based therapy, and includes viral load and genotypic resistance testing, along with urine collection. Using these stored urine specimens, we will assess the validity of a recently developed urine TFV immunoassay to differentiate between adherence and resistance-based treatment failure in sub-Saharan Africa through the following specific aims: 1) We will validate the performance of the urine TFV immunoassay compared to a reference standard of liquid chromatography-tandem mass spectrometry among participants on TFV-containing ART regimens in the REVAMP study. 2) We will determine the diagnostic performance of the urine TFV immunoassay to distinguish virologic failure with wild-type virus from virologic failure with resistant virus in participants failing TFV-containing first-line regimens. Data generated from these aims will be used to finalize development of a urine TFV point-of-care adherence assay for treatment, which we will propose to evaluate in an NIH R01-funded trial to differentiate between adherence and resistance-driven treatment failure in RLS.